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POWER GENERATION AND TRANSMISSION
67
in by compressed air at high pressure. If it is forced in suddenly, the engine is of the "high-pressure" or "semi-Diesel" type, although it uses the Otto rather than the Diesel cycle. If the oil delivery is gradual, the Diesel engine results: in either case maximum pressures of 500 to 600 Ib. are realized.
Carburetor Engines.  The gasoline engine is the type of all of these, the fuel being mixed with air by vaporization and agitation, and the mixture drawn into the engine by piston suction. The heat value of gasoline is close to 18,320 + 40(  10) B.t.u. per pound where B = Baum6 hydrometer reading. The diagram shows the low vaporizing temperature of gasoline at a given pressure. Kerosene, as indicated, vaporizes with much less readiness. It therefore requires a heated carburetor. This implies complication in starting. It also requires a limitation of compression or water injection to prevent preignition. The alcohols vaporize with moderate ease. Ethyl alcohol of 95 per cent volumetric strength has a heat value of 10,880 B.t.u. per pound (low). For kerosene, B.t.u. per pound -18,440 + 40(B - 10).                                                        fuels.
FlG. 29.
liquid
USUAL COMPRESSION PRESSURES FOR INTERNAL-COMBUSTION ENGINES
Fuel	Type of engine	Range of compression, Ib. per sq. in. gage	Average compression in Ib. per sq. in. gage
Gasoline	Automobile                    ........	45-95	65
Gasoline                       . .	Stationary            ............	60-105	70
Kerosene            ......	Vaporized before entering cylin-		
	der	451-852	65
Alcohol    .............	Vaporized before entering cylin-		
	der                                   ......	120-210	150
			
1 With hot mixture without water injection.
2 With water injection.
(From MARKS' Mechanical Engineers' Handbook)
All of these engines are of importance in small sizes only, where the high cost of fuel used can be tolerated. They have jump-spark or make-and-break electric ignition. Mean effective pressures for gasoline are 70 to 85 Ib. at 65 Ib. compression (absolute) and 85 to 95 Ib. at 100 Ib. compression. For kerosene, 50 to 65 Ib. mean effective pressure is realized at the lower compression, 70 to 85 Ib. at 115 Ib. compression. The equation for horsepower given on page 63 applies to these engines. The ideal efficiency is also as there indicated, and the actual efficiency usually about half the ideal. Jacket and exhaust heat constitute the two large losses. Air-cooling is practicable in small sizes and unimportant applications-